1. Field of the Invention
This invention relates to a method of and a device for charging by the use of corona discharge. Charging by the use of corona discharge will hereinafter be described with electrophotography as an example. The electrophotographic processes generally include a method whereby charge of the positive or the negative polarity is applied to a two-layer photosensitive medium comprising a photoconductive layer and a conductive base and subsequently the photosensitive medium is exposed to image light to form thereon an electrostatic latent image which is in turn subjected to the developing step to provide a visible image, and a method whereby primary charge of the positive or the negative polarity is imparted to a three-layer photosensitive medium comprising a transparent insulating layer, a photoconductive layer and a conductive base and subsequently image light and secondary charge are applied to the photosensitive medium to remove the primary charge and form an electrostatic latent image thereon, whereafter the photosensitive medium is subjected to whole surface exposure to increase the contrast of the latent image, which is then subjected to the developing step to provide a visible image. This latter process is shown in FIG. 1 of the accompanying drawings, wherein reference character 1 designates a photosensitive medium rotatable in the direction of the arrow, 2 a primary charger, 3 an image light, 4 a secondary charger, 5 a light source for whole surface exposure, 6 a developing device, 7 an image transfer charger for facilitating the image transfer to transfer paper 8, and 9 designates a cleaning device. The charging used in these electrophotographic processes utilizes the DC corona discharge or the AC corona discharge and it is known, for example, that the DC corona discharge is utilized for the primary charger 2 and the image transfer charger 7 and the AC corona discharge is utilized for the secondary charger.
2. Description of the Prior Art
An example of the charger according to the prior art is illustrated in FIG. 2(a), wherein reference numeral 21 designates a high voltage source, 22 a corona discharge wire and 1 a photosensitive medium. The high voltage source 21 may be either an AC voltage source or a DC voltage source, and a voltage greater than the corona discharge start voltage VC may be applied therefrom to the corona discharge wire 22 to produce a corona discharge current which may impart charge to the surface of the photosensitive medium.
An important point in electrophotography or the like is that a constant surface potential should be stably provided to ensure good reproducibility of the electrostatic latent image thus produced. Corona charge greatly affects the electrostatic latent image and therefore, in order to stabilize the surface potential, it is necessary in the charger of FIG. 2(a) that various factors, such as the relative moving velocity of the photosensitive medium and the corona discharger, the width of the opening of the corona discharger (formed by the shield 22S), the distance between the corona discharge wire and the photosensitive medium, atmospheric conditions such as temperature, humidity, etc., and the voltage applied be constant at all times.
FIGS. 2(b) to 2(d) show conventional chargers designed to reduce the variation in surface potential which may result from changes of the above-mentioned factors. In FIG. 2(b), a resistor 24 is serially inserted in the high voltage output side of the voltage source 21; in FIG. 2(c), the output of the voltage source may be divided by rectifiers 26-1 and 26-2 while a resistor 24 is inserted and connected to the corona wire 22; in FIG. 2(d), a constant voltage discharge tube 25 is employed instead of the resistor 24. In any of these, the change in corona discharge resistance resulting from the change in atmospheric conditions or from the irregularity of the distance between the corona discharge wire and the surface of the photosensitive medium could not sufficiently be compensated for and thus, the stability of the resultant surface potential and of the finally obtained visible image has been unsatisfactory. For example, the change of atmospheric conditions from normal temperature and humidity to high temperature and humidity led to an unfavorable result in that the developed visible image was fogged.